Method for producing non-aging silicon-killed steel



Nov. 25, 1958 w. c. LESLIE METHOD FOR PRQDUCING NON-AGING SILICON-KILLEDSTEEL Filed Jan. 15, 1954 FIE- 1- m 8 .N -QSEQW 57m l, wa f METHOD FORPRODUCING NON-AGING SILICON-KILLED STEEL William C. Leslie, Cleveland,Ohio, assignor to United States Steel Corporation, a corporation of NewJersey This invention relates to the reduction of strain-agingtendencies of silicon-killed steel and more particularly to a method ofheat treating low-carbon, silicon-killed steel to substantiallyeliminate subsequent strain aging thereof.

There has been considerable confusion in the literature and among theworkers in the field concerning the cause of strain aging of W-carbon,i. e. less than .30% carbon, steels. However, it is now well establishedthat the primary cause thereof is nitrogen in solid solution in ferrite.While carbon also affects strain aging, nitrogen is much more effectivebecause of its greater solubility in ferrite at low temperatures. Theonly feasible method proposed to date of removing this nitrogen from thesolution is by alloying the steel with elements which combine withnitrogen to form stable nitrides. Elements which have been successfullyused for this purpose are strong deoxidizers, such as aluminum,titanium, vanadium and zirconium. Silicon is a common and inexpensivedeoxidizer used to produce killed steels point elongation, removing themfrom the testing machine, and aging in boiling water. After aging, thespecimens were replaced in the tensile testing machine and pulled tofracture. The extent of the strain aging was determined by subtractingthe stress at the point at which the load was released (A) from theyield point after aging (B) dividing by A and expressing as a percentagethus B-A A Percent strain aging: X100 Steel 1 Percent Carbon .20Manganese .80 Phosphorus .01 1 Sulphur .018 Silicon .18 Aluminum .004Nitrogen .007

This steel was killed by the addition of ferrosilicon in the furnacewith slight aluminum additions in the mold.

Specimens of this steel as shown in Figures 1 and 2 of the drawings wereaustenitized at 2250 F. and 1600 F. followed by air or furnace coolingto room temperature. The tests showed that the strain aging of thissteel was substantially unchanged by this treatment, re-

Unid States Patent() maining above 10%. Similar resultsrwerev obtainedwith specimens from another heat of steel identified as Steel 2 havingthe following analysis:

Steel 2 Percent Carbon .25 Manganese .50 Phosphorus .013 Sulphur .027Silicon .23 Aluminum Y .005 which was killed similarly to Steel 1.Specimens thereof along with further specimens of Steel 1 were tested byheating to 1600 F. followed by cooling to room temperature at a rate of1315 F. As in the foregoing experiment, the strain-aging characteristicsof these specimens were relatively unchanged, remaining above 10%.

Contrary to the conclusion to be drawn from the foregoing thatsilicon-killed steel is unaffected by the foregoing treatments, I havediscovered that silicon-killed steels can be rendered substantiallynon-aging by holding for extended periods of time, i. e. over one hourat temperatures well below the A1 temperature of the steel at atemperature between 1000 and 1200 F.

As a demonstration of my preferred treatment, specimens of Steel 1 wereagain heated to 2250 F. or 1600 F. and then cooled to 1100 F. and heldthere for periods of time with the results shown in Figures 1 and 2.Samples of Steel 2 were similarly treated after heating to 1600 F. byholding at 1l00 F. for the times shown on Figure 2.

As clearly shown by the graphs of Figures 1 and 2 holding for timesbeyond one hour at 1100 F. substantially reduced the strain-agingcharacteristics of the specimens and that such characteristics werereduced to 2% or less by holding at such temperature for 24 hours.

This minoror residual amount of strain aging is`consideredinconsequential.

Tests were also conducted by holding specimens of Steel 1 at 1300" F.for extended periods but as shown on the drawings such treatment had noeffect on the strain-aging characteristics.

While I have shown and described two specific embodiments of myinvention, it Will be understood that these embodiments are merely forthe purpose of illustration and description and that various other formsmay be devised within the scope of my invention, as defined in theappended claims.

I claim: v

l. A method of reducing the strain-.aging characteristics ofmanganese-containing silicon-killed steel characterized by substantialfreedom from the strong deoxidizers aluminum, titanium, vanadium andzirconium, with the balance iron and residual elements in common amountscomprising holding the `steel ata temperature above about l000 F. andbelow about 1200 F. for at least one hour.

2. A method of reducing the strain-aging characteristics ofmanganese-containing silicon-killed steel characterized by substantialfreedom from the strong deoxidizers aluminum, titanium,A vanadium andzirconium, with the balance iron and residual elements in common amountscomprising austenitizing the steel, then cooling it to a temperatureabove about l000 F. and below about l200 F. and holding it at suchtemperature for .at least one hour.

3. A method of substantially eliminating the strainaging tendency ofmanganese-containing low-carbon, silicon-killed steel characterized bysubstantial freedom from the strong deoxidizers aluminum, titanium,vanadium and zirconium, with the balance iron and residual elements incommon amounts comprising holding the steel at about ll00 F. for atleast one hour until the strain-aging tendency is reduced to below 2% 4.AY method of substantially eliminating the strainaging characteristicsof manganese-containing low-carbon, silicon-killed steel characterizedby substantial freedom from the strong deoxidizers aluminum, titanium,vanadium and zirconium, with the balance iron and residual elements incommon amounts comprising austenitizing the steel, cooling it to about1100 F. .and holding it at said temperature for at least one hour untilthe strain-aging tendency is reduced topless than 2% 5. A method ofsubstantially eliminating the strainaging characteristics ofmanganese-containing low-carbon, silicon-killed steel characterized bysubstantial freedom from the strong deoxidizers aluminum, titanium,vanadium and zirconium, with the balance iron 4and residual elements incommon amounts comprising austenitizing the steel, cooling it to about1100 F. and holding it at said temperature for about 24 hours whereby toreduce the strain-'aging tendency to less than 2% 6. A method ofsubstantially eliminating the strainagingcharacteristics of low-carbon,silicon-killed steel containing .30% maximum carbon, 1.50% maximummanganese, .50% maximum silicon with the balance iron and residualelements in common amounts comprising austenitizing said steel, coolingit to about 1100 F. and holding it at said temperature for about 24hours, whereby to reduce its strain-aging tendency to less than 2% 7. Amethod of substantially eliminating the strain aging characteristics oflow-carbon, silicon-killed steel containing .30% maximum carbon, 1.50%maximum manganese, .50% maximum silicon with the balance iron andresidual elements in common amounts comprising austenitizing said steel,cooling it to between 1000 and 1200 F. and holding it at saidtemperature until its tendency to strain age has been reduced to lessthan 2%.

References Cited in the ile of this patent Metals and Alloys, vol. 5,May 1934, pages 11G-1712. Atlas of Isothermal Transformation Diagrams,publ. by U. S. S. Corp., 1951, pages 77, 79.

UNITED STATES PATENT OFFICE Certificate of Correction Patent No.2,861,609 November 25, 1958 William C. Leslie It is hereby certifiedthat error appears in the printed specification of the above numberedpatent requiring correction and that the said Letters Patent should readas corrected below.

Column 1, line 34., after steels insert the following:

and it 'also has the property of combining with nitrogen to form astable compound. Despite this latter fact, silicon-killed low-carbon`steels are generally susceptible to strain aging.

It is accordingly an object of this invention to produce silicon-killedlowcarbon steel which is substantially free from the tendency to strainage.

The foregoing and further objects will be apparent from the followingspeciication when read in conjunction with the attached drawing whereinFigures 1 and 2 are graphs wherein percent of strain aging is plottedagainst a time log scale for heat treatments as indicated thereon.

We have discovered that silicon-killed low-carbon steel can be renderedsubstantially free from strain aging by a pro erly conducted heattreatment as hereinafter described. The steel under consideration can begenerally defined as containing .30% maximum carbon, 1.50% maximummanganese, .50% maximum silicon with the balance substantially iron andresidual elements or impurities in common amounts. Since carbon is asecondary cause of strain aging, the steels under consideration cannotbe rendered completely non-aging under all conditions as high a ingtemperatures will cause slight strain a ing. To produce a steelcompletely Free from strain agin would require the addltion of elementswhich reduce the solubility of both calgbon and nitrogen in ferrite to avery low level.

Y In my work on strain aging in connection with this invention, theextent of strain aging was determined y tensile tests. Thus the strainaging charactlristicdof samples were determined by straining tensiletest specimens through t e yie Signed and sealed this 10th day of March1959.

[smh] Attest: v

KARL H. AXLINE, l ROBERT C. WATSON, Attestng Ocer. Commissioner ofPatents.

1. A METHOD OF REDUCING THE STRAIN-AGING CHARACTERISTICS OFMANGANESE-CONTAINING SILICON-KILLED STEEL CHARACTERIZED BY SUBSTANTIALFREEDOM FROM THE STRONG DEOXIDIZERS ALUMINUM, TITANIUM, VANADIUM ANDZIRCONIUM, WITH THE BALANCE IRON AND RESIDUAL ELEMENTS IN COMMON AMOUNTSCOMPRISING HOLDING THE STEEL AT A TEMPERATURE ABOVE ABOUT 1000*F. ANDBELOW ABOUT 1200*F. FOR AT LEAST ONE HOUR.